javax.servlet.jsp.tagext
Interface Tag

The interface of a classic tag handler that does not want to manipulate
its body. The Tag interface defines the basic protocol between a Tag
handler and JSP page implementation class. It defines the life cycle
and the methods to be invoked at start and end tag.

Properties

The Tag interface specifies the setter and getter methods for the core
pageContext and parent properties.

The JSP page implementation object invokes setPageContext and
setParent, in that order, before invoking doStartTag() or doEndTag().

Methods

There are two main actions: doStartTag and doEndTag. Once all
appropriate properties have been initialized, the doStartTag and
doEndTag methods can be invoked on the tag handler. Between these
invocations, the tag handler is assumed to hold a state that must
be preserved. After the doEndTag invocation, the tag handler is
available for further invocations (and it is expected to have
retained its properties).

Lifecycle

Lifecycle details are described by the transition diagram below,
with the following comments:

[1] This transition is intended to be for releasing long-term data.
no guarantees are assumed on whether any properties have been retained
or not.

[2] This transition happens if and only if the tag ends normally
without raising an exception

[3] Some setters may be called again before a tag handler is
reused. For instance, setParent() is called if it's
reused within the same page but at a different level,
setPageContext() is called if it's used in another page,
and attribute setters are called if the values differ or are expressed
as request-time attribute values.

Once all invocations on the tag handler
are completed, the release method is invoked on it. Once a release
method is invoked all properties, including parent and
pageContext, are assumed to have been reset to an unspecified value.
The page compiler guarantees that release() will be invoked on the Tag
handler before the handler is released to the GC.

Empty and Non-Empty Action

If the TagLibraryDescriptor file indicates that the action must
always have an empty action, by an <body-content> entry of "empty",
then the doStartTag() method must return SKIP_BODY.

Otherwise, the doStartTag() method may return SKIP_BODY or
EVAL_BODY_INCLUDE.

If SKIP_BODY is returned the body, if present, is not evaluated.

If EVAL_BODY_INCLUDE is returned, the body is evaluated and
"passed through" to the current out.

getParent

The getParent() method can be used to navigate the nested tag
handler structure at runtime for cooperation among custom actions;
for example, the findAncestorWithClass() method in TagSupport
provides a convenient way of doing this.

The current version of the specification only provides one formal
way of indicating the observable type of a tag handler: its
tag handler implementation class, described in the tag-class
subelement of the tag element. This is extended in an
informal manner by allowing the tag library author to
indicate in the description subelement an observable type.
The type should be a subtype of the tag handler implementation
class or void.
This addititional constraint can be exploited by a
specialized container that knows about that specific tag library,
as in the case of the JSP standard tag library.

doStartTag

Process the start tag for this instance.
This method is invoked by the JSP page implementation object.

The doStartTag method assumes that the properties pageContext and
parent have been set. It also assumes that any properties exposed as
attributes have been set too. When this method is invoked, the body
has not yet been evaluated.

This method returns Tag.EVAL_BODY_INCLUDE or
BodyTag.EVAL_BODY_BUFFERED to indicate
that the body of the action should be evaluated or SKIP_BODY to
indicate otherwise.

When a Tag returns EVAL_BODY_INCLUDE the result of evaluating
the body (if any) is included into the current "out" JspWriter as it
happens and then doEndTag() is invoked.

BodyTag.EVAL_BODY_BUFFERED is only valid if the tag handler
implements BodyTag.

The JSP container will resynchronize the values of any AT_BEGIN and
NESTED variables (defined by the associated TagExtraInfo or TLD)
after the invocation of doStartTag(), except for a tag handler
implementing BodyTag whose doStartTag() method returns
BodyTag.EVAL_BODY_BUFFERED.

Returns:

EVAL_BODY_INCLUDE if the tag wants to process body, SKIP_BODY
if it does not want to process it.

doEndTag

Process the end tag for this instance.
This method is invoked by the JSP page implementation object
on all Tag handlers.

This method will be called after returning from doStartTag. The
body of the action may or may not have been evaluated, depending on
the return value of doStartTag.

If this method returns EVAL_PAGE, the rest of the page continues
to be evaluated. If this method returns SKIP_PAGE, the rest of
the page is not evaluated, the request is completed, and
the doEndTag() methods of enclosing tags are not invoked. If this
request was forwarded or included from another page (or Servlet),
only the current page evaluation is stopped.

The JSP container will resynchronize the values of any AT_BEGIN and
AT_END variables (defined by the associated TagExtraInfo or TLD)
after the invocation of doEndTag().

release

void release()

Called on a Tag handler to release state.
The page compiler guarantees that JSP page implementation
objects will invoke this method on all tag handlers,
but there may be multiple invocations on doStartTag and doEndTag in between.